In the late 1970s, a pilot living near the Colville River in northern Alaska started seeing the trails of snowshoe hares. He “didn’t believe it at first,” he wrote later in an email, since he hadn’t spotted any hares since he began monitoring the area a few years ago. But within the next five years, he wrote, “the trails were so hard packed I could almost walk on them.”

Snowshoe hares have become common on the North Slope of Alaska over the last few decades, and now scientists believe they know why. Climate change has allowed shrubs to grow taller, providing new habitat where hares can forage and hide from predators.

The researchers studied records of peak river discharge for four rivers in northern Alaska dating back to the 1970s. In 2012, the team visited 10 sites by the Noatak River and Dalton Highway, measured the shrubs, and searched for signs that hares had nibbled on the plants. The researchers also measured some particularly tall shrub patches by the Chandler and Colville Rivers.

The date of peak river discharge in the spring has crept an average of 3.4 days earlier per decade since the 1980s, the authors report in Global Change Biology. That means floodplain plants have a longer growing season. At the Chandler and Colville Rivers, the change in growing season enabled shrubs to grow 78 percent taller from 1960 to 2010, the team estimates.

The researchers’ observations suggest that snowshoe hares need shrubs to be at least 1.24 to 1.36 meters tall to provide suitable habitat. The shrubs in the area likely reached that height sometime between 1964 and 1989, allowing the hares to move in. The team didn’t find any evidence that hares lived on the North Slope before the 1970s.

The emergence of tall shrubs could invite other animals such as songbirds to move northward as well. “The increase in shrub habitat and associated herbivores advancing toward the Arctic Coast as a result of 20th century warming,” the authors write, “is the contrasting terrestrial counterpart to the decline in sea ice and associated marine mammals.” — Roberta Kwok | 12 November 2015